1. Field of the Invention
This disclosure relates to formation density logging, and more particularly to methods and apparatus for determining the densities of earth formations surrounding a borehole that compensate for the effects of mudcake and standoff.
2. Background of the Invention
A probe, known as a sonde, is lowered into an oil well borehole and used to determine the formation density surrounding the borehole. Formation density is used by Geologists to predict the depth levels of productive flows of oil. Conventional logging sondes include a source of gamma rays (or X-rays), at least one detector and a material that shields the detector from the radiation source to prevent counting of radiation emitted directly from the source. During operation of the sonde, gamma rays are emitted from the radiation source and travel through the wall of the borehole and into the formation to be studied. These gamma rays interact with the atomic electrons of the formation; and in the Compton scattering process, some of the photons emitted from the source into the formation are scattered toward the detector in the logging tool. Since the number of the gamma rays returning to the detector depends on the formation density, the formation density is thus calculated based on the count rate or the intensities of the detected gamma rays.
In the prior art, Betatrons have been known as sources of X-ray emissions. U.S. Pat. No. 5,525,797 further disclosed a Betatron having an internal target effective in generating high-intensity X-rays. This patent is incorporated by reference herein in its entirety.
One problem affecting the accuracy of formation density measurement is the mudcake and tool standoff effect. Mudcake is a layer of solid material consolidated from drilling fluid that exists along the outer periphery of the borehole. And standoff is the distance from the sonde to the borehole wall. Generally, the density of the mudcake is different from the density of the formations. As gamma rays have to traverse the mudcake twice before they reach the detector and the detector only measures the count rate or intensity of the detected gamma rays, the formation density obtained is very inaccurate if the mudcake effect is not compensated for.
Traditionally, at least two detectors are used to compensate the standoff caused by mudcake build-up or tool standoff. The basic two detector sonde includes a detector close to the radiation source (short spaced detector SS) and a detector further away from the radiation source (long spaced detector LS). The SS detector has a shallow depth of investigation and is more sensitive to the borehole fluid or mudcake between the sonde and the formation, while the LS detector has a deeper depth of investigation and is less sensitive to the borehole environment and more sensitive to the formation.
A “spine-and-ribs” method is generally used to correct the LS detector measurements for tool standoff in a two detector logging tool. The “spine” is the locus of the count rates of the LS detector and the SS detector at different formation densities without mudcake. “Ribs” extend from the spine and each of them is formed by connecting the LS detector and the SS detector count rate data points for a fixed formation density and various mudcake thicknesses. In a logging run, a cross-plot location corresponding to a point on a particular rib is found by using the count rates from the LS detector and the SS detector. By tracing this rib back to the spine, the formation density is determined as indicated by the intersection of the rib and spine. Reference may be made to Ellis, D. V., Well Logging for Earth Scientists, Springer, 2nd edition, 2007, for a detailed discussion of a formation density measurement using two detector logging tools.
To further improve the accuracy of the formation density calculated, three or more detectors sometimes are used. U.S. Pat. No. 5,912,460 disclosed a method for determining formation density with three or more detectors to compensate large standoff. This patent is incorporated by reference herein in its entirety.
In the traditional multiple detector well logging tool, the SS detector is very close to the radiation source which makes it difficult to shield the detector from direct radiation from the source. In addition, when more detectors are used, the cost of the logging tool increases significantly. Therefore, it is an aspect of this invention to avoid using the SS detector, reduce the total number of the detectors used, while at the same time keep the measurements of formation densities accurate and reliable. It is another aspect of this invention to provide more information about the formation and standoff conditions when multiple detectors are used.